Insulating coating for transformer wires

ABSTRACT

Improved three component insulating coatings for both wet solvent and dry electrostatic application to transformer wires contain an increased epoxy resin concentration and a decreased amount of polyvinyl acetal and phenol aldehyde resins. The increased epoxy content gives substantially improved coatings with no adverse dissipation and hydrolytic effects. The epoxy constituent can range from 30-60 percent by weight providing the ratio of the polyvinyl acetal to phenol aldehyde is kept within the range of from 1 to 1 to 2 to 1.

BACKGROUND OF THE INVENTION

This invention relates to enameling compositions used as an insulatingcoating material for transformer wire. One of the problems involved withcurrent methods and materials used in manufacturing and applyingtransformer wire coatings is the need for a suitable solvent fordissolving the coating constituents and providing a low viscositysolution. Since the solvents currently utilize expensive hydrocarbon andcresols, other materials are continuously being evaluated in an attemptto reduce the quantity of solvents employed. Over the past 15 years forexample, an insulating coating composition consisting of a mixture ofpolyvinyl acetal and phenolic were reduced in solvent content from 85weight percent down to 75 percent. This reduction was realized byvariations in the polyvinyl acetal and phenolic materials as well as aselected combination of hydrocarbon and cresol solvents. Besides theexpense involved in utilizing liquid solvents in the wire coatingindustry, requirements are now being made by the EnvironmentalProtection Agency to reduce solvent usage by a substantial amount inorder to reduce the overall concentration of solvents existing in theatmosphere.

U.S. Pat. application Ser. No. 595,034 filed July 11, 1975, nowabandoned, discloses a three component wire insulating composition whichincludes an epoxy resin in combination with polyvinyl acetal andphenolic resins. The three component composition further reduced thesolvent content down to 70 weight percent by taking advantage of thegood filmforming properties of the epoxy resin. The use of an epoxy wirecoating per se has not heretofore proven feasible due to the poorhydrolytic stability existing with known epoxy compounds. Whentransformer wires are coated for electrical insulating purposes, and aresubjected to long exposure times in the presence of heat and moisture,it is essential that the coating remain electrically stable. Hydrolyticstability therefore is an important parameter for evaluating efficienttransformer wire insulating materials. In order to determine hydrolyticstability, the transformer wire coatings are subjected to moisture andtemperature for a prescribed period of time and are subsequentlymeasured to determine whether the electrical insulating properties havedeteriorated. Wires coated with epoxy compounds per se becomehydrolytically unstable and are infeasible for long term transformerwire coatings.

Another requirement for transformer wire coating materials is a lowdissipation factor. Since the electrical properties of the coatingdepend to a large extent upon the transformer operating temperature, thewire coating materials must be able to withstand the high temperaturesinvolved, under short circuit load conditions. In order for thetransformer wire coating to be electrically and thermally stable, thedissipation factor, which is a fairly good indication of the ability ofthe coating to dissipate heat, must be determined at various operatingtemperatures. If the transformer wire coating has too high a dissipationfactor, thermal runaway can occur causing insulation to decrease to aninoperable value.

Formulations intended for use as insulating coatings must be carefullyevaluated for temperature, moisture and overall electrical stability forlong periods of time in order to ensure that short circuits do not occurdue to electrical insulation failure. The polyvinyl acetal and phenoliccomposition disclosed within the aforementioned U.S. Patent applicationcontains approximately phenolic resin in a ratio of one part to twoparts polyvinyl acetal. Attempts to increase the polyvinyl acetalconcentration resulted in wire coatings having too high a dissipationfactor along with hydrolytic instability. Attempts on the other hand toincrease the phenolic content seriously interfered with the flexibilityof the coating. As described earlier, various epoxy resin compositionsprovided good flexible and pin hole free insulating coatings but werehydrolytically unstable and unsuitable per se as wire coatings. Attemptsto combine epoxy resins, phenolic resins, and polyvinyl acetal such assuggested within U.S. Pat. No. Re 25,625 have not proven successful whenevaluated for transformer wire coatings. Coatings prepared from theaforementioned re-issued patent disclosure were too inflexible towithstand the transformer winding operation. Wire coatings prepared fromthe adhesive composition disclosed within U.S. Pat. No. 3,239,598resulted in wire coatings having an excessive dissipation factor andpoor flexibility.

The three component coating composition disclosed within aforementionedU.S. Patent application Ser. No. 595,034 resulted in wire coatingshaving good flexibility, low dissipation factor and hydrolyticstability. The addition of epoxy resin to the polyvinyl acetal andphenolic resins substantially improved the flow properties of thecoating during the coating process. With ratios of polyvinyl acetal tophenolic from about 2 to 1, additions of about 11 to 25% epoxy resin canbe employed. It has since been discovered that even better coatings canbe obtained with further increases in the amount of epoxy added to thecoating composition. The higher epoxy compositions allow the coating toflow more evenly over the wire surface when applied by dry electrostatictechniques and, is a valuable feature in transformer wire coatingoperations. These higher epoxy compositions permit the formulation ofhigher solids enamels which greatly reduce the solvent required.

The purpose of this invention therefore is to provide three componentwire coating composition having a high concentration of epoxy resin.

SUMMARY OF THE INVENTION

Improved transformer wire insulating compositions are disclosed havingincreased amounts of epoxy resin. The increased epoxy constituentsubstantially improves the coating flow properties and provides forwider variations in the coating application process. Proportionatelydecreasing the phenolic and polyvinyl acetal resins allows for theincreased epoxy constituent without affecting the coating electricalproperties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a transformer wire coated with anenamel composition according to the invention;

FIG. 2 is a triaxial diagram of some state of the art wire coatingcompositions;

FIG. 3 is a graphic representation of the relationship betweendissipation factor and PVA to phenolic ratio for constant epoxyconcentrations;

FIG. 4 is a graphic representation of the relationship between the ratioof PVA to phenolic for operable concentrations of epoxy resin; and

FIG. 5 is a triaxial diagram of the improved coating composition of theinvention.

GENERAL DESCRIPTION OF THE INVENTION

The aforementioned U.S. Patent application discloses the combination ofthe reaction product of a mixture of polyvinyl acetal resin, phenolaldehyde resin and epoxy resin in a particular range of proportions forelectrically insulating coatings for transformer wires. Transformer wirecoatings to be subjected to the high temperatures and moistureconditions existing within a transformer enclosure must have a lowdissipation factor, good flexibility and be hydrolytically stable forthe reasons discussed earlier. The aforementioned U.S. Patentapplication limited the ranges of the three constituents to ensure thatno problem in flexibility, dissipation factor or hydrolytic stabilitywould occur. The phenol aldehyde resin was kept at less than 40% inorder not to reduce the flexibility and at least 20% in order to avoidan excessively high dissipation factor. The polyvinyl acetal resin waskept at less than 65% in order not to create an excessively highdissipation factor while at least 40 percent was required to providesatisfactory flexibility. The epoxy concentration was kept at less than30 percent to avoid both high dissipation factor problems as well ashydrolytic instability. Seven percent of the epoxy was required howeverto promote adequate fusion of the powder particles and to impartuniformity to the coating. Since the epoxy constituent is an extremelybeneficial contributor to the amount of solids remaining on the wire,after the fusion process, attempts to increase the epoxy content above30 percent were heretofore infeasible because of the high dissipationfactor and hydrolytic instability inherent within the epoxy material.

Since the aforementioned three component wire coating composition isapplied to transformer wire, by a wet die process, as well as by a dryelectrostatic coating process, tests were undertaken to determine thetheoretical maximum solids content of coating material that can beoperably applied by both methods to transformer wire. The wet dietechnique was chosen as the test method of application for purposes ofconvenience in applying to the wire. After dissolving samples containingincreasing amounts of epoxy resin, plastic films were case from thesamples. Hydrolytic stability and dissipation factor measurements werealso taken in order to determine whether increases in the amount ofepoxy within the coating could be tolerated without seriouslyinterfering with the electrical characteristics. Using the resins andsolvents disclosed within the aforementioned U.S. Patent application itwas then discovered that for very high epoxy coatings containingapproximately the same amount of phenolic resin as described within theteachings of the aforementioned U.S. Patent application andsubstantially less polyvinyl acetal resin, strong, flexible andhydrolytically stable coatings resulted having satisfactory lowdissipation factors.

FIG. 1 shows the insulation coating 2 on the transformer wire 1containing ingredients described within the aforementioned U.S. PatentApplication in amounts ranging from low to high epoxy resin content.

FIG. 2 shows the preferred range of compositions B as disclosed thereinalong with the operational range A. It is to be noted that highconcentrations of polyvinyl acetal increases the dissipation factor toan excessive and inoperable value as indicated and that highconcentrations of phenol aldehyde produce coatings having poorflexibility. The high ranges of epoxy are known to produce problems withhydrolytic stability. The compositional range disclosed within theaforementioned U.S. re-issue patent is shown at C and can be seen toencompass high dissipation problems at the high polyvinyl acetal end ofthe range and problems with hydrolytic stability at the high epoxy end.The composition disclosed within the aforementioned U.S. patent is shownat D to exhibit poor flexibility when evaluated as a wire coatingmaterial.

DESCRIPTION OF THE PREFERRED EMBODIMENT

For the increasing epoxy compositions described earlier and evaluatedfor dissipation factor, hydrolytic stability, and flexibility, it wasdiscovered that the dissipation factor increased with increasing ratiosof polyvinyl acetal to phenolic as well as increasing amounts of epoxyresin. When the epoxy content was kept constant and the amount ofpolyvinyl acetal was increased relative to the phenolic resin thedissipation factor also increased in proportion to the polyvinyl acetalto phenolic ratio. The relationship between dissipation factor andpolyvinyl acetal to phenolic ratio (P.V.A./phenolic) is shown at E inFIG. 3. Since the low ratio values exhibited low dissipation for thesame epoxy content of 35 percent, an attempt was made to increase theepoxy content in excess of 35% to determine whether low dissipationcould be realized within the same range of ratios. In order to determineoperability as an insulating coating the increased epoxy resins werealso evaluated for flexibility and hydrolytic stability using standardtechniques. The samples which showed good hydrolytic stability and goodflexibility as well as a dissipation factor of less than 25 percent at170° C. were considered to pass the evaluation. Coatings havingdissipation factors in excess of 25 percent at 170° C. and/or havingpoor electrolytic stability and poor flexibility failed the evaluation.

The results of the large series of evaluations for increasing epoxymixes within the ratio of P.V.A. to phenolic range of 1.0 to 2.0 isshown in FIG. 4.

The samples which failed any of the aforementioned parameters areindicated by crosses and the samples which passed all the parameters areindicated by circles to show the effective range of both the epoxycontent and the P.V.A. to phenolic ratios. An approximate compositionalrange of epoxy mixes is designated at F where the epoxy content variesfrom as low as 10 percent to as high as 60 percent within a range inP.V.A. to phenolic ratios of from 1.10 to 1.90. It is to be noted thatboth low and high epoxy compositions failed within the range of ratiosand that low and high ratios failed for the same epoxy compositions.

FIG. 5 shows the effective compositional range G for concentrations ofepoxy, phenol aldehyde and polyvinyl acetal as measured in weightpercent for the samples from FIG. 4 that possessed the necessaryrequirements for operable insulating wire coatings. The increase in theoverall amounts of the epoxy constituent expands the manufacturingtolerances for the process of preparing and applying the coatingcomposition and greatly improves over the low epoxy composition shownearlier in FIG. 2. The increased epoxy content should exhibitanticipated problems in hydrolytic stability since the ranges nowindicated at G in FIG. 5 extend within the area of the diagram withinFIG. 2 where hydrolytic instability occurs.

The improved compositions within the region designated as F having theconfiguration of an inverted truncated cone, however, do not exhibitpoor flexibility, hydrolytic instability or high dissipation asanticipated from the teachings of the prior art. Since the improvedcompositional range now comprises: Polyvinyl acetal 20-46% by weight;phenolic resin 14-34% by weight; and epoxy resin 30-60% by weight,insufficient polyvinyl acetal should therefore result in wire coatingshaving poor flexibility as indicated in FIG. 2. The excellent hydrolyticstability and low dissipation factor for the high epoxy-low polyvinyl toacetal composition implies a coaction between the epoxy resin and thepolyvinyl acetal resin since the phenolic resin composition remainsrelatively unchanged. Since the epoxy material is hydrolyticallyunstable per se it is surprising, therefore, that by decreasing theparticular component of the composition that improves the necessaryproperty of hydrolitic stability (polyvinyl acetal) and increasing thecomponent which has high inherent dissipation (epoxy) can result in awire coating possessing superior electrical insulating properties.

Compositions were prepared using the resin materials described withinthe forementioned U.S. patent application but having the improved rangesshown in FIG. 5 and were applied to transformer wires by a wet floatingdie process in one case and by a dry electrostatic process in another.In both methods of application the resulting coatings exhibiteddissipation factors less than 25 percent at 170° C. with good continuousand flexible coatings that were also hydrolytically stable. In thesolvent application system, the solvent represented approximately 60percent by weight of the total solution whereas the composition appliedby entraining the dry powder in a fluidized bed gas stream and applyinga high voltage electrostatic DC field between the powder and thetransformer wire equally adhered without any solvents at all. The wirewas heated to a first temperature to fuse the coating and then to asecond higher temperature to cause the resins to react.

Besides providing good rheological flow properties to the coating bydecreasing the polyvinyl acetal and increasing the epoxy resin contentimpressive costs savings can also be realized since the polyvinyl acetalis the most expensive ingredient in the composition. Combining thecompositional range of the prior art with that of the improved coatingformulation provides a large range of manufacturing latitude and greatlyimproves the overall transformer manufacturing efficiency. Extending therange from 20 to 65 parts by weight polyvinyl acetal; 14 to 40 percentby weight phenol aldehyde resin; and 7 to 60 weight percent epoxy resingreatly expands the overall manufacturing tolerances. In the transformerwire coating field the range in materials composition determine themanufacturing "window". It can be readily understood, therefore, thewider the range of materials used in preparing the wire coatingcomposition the larger the manufacturing window and, ultimately, thelower the manufacturing costs.

Although the improved wire enamel composition of the invention isdisclosed for use within power transformers this is by way of exampleonly. The improved wire coating composition of the invention and themethods of application thereof find application wherever electricallyinsulating wire coatings may be required having good flexibility, lowdissipation and good hydrolitic stability.

What is claimed is:
 1. An improved insulating coated transformer wire of the type consisting of a wire having an insulating coating of phenol aldehyde resin, polyvinyl acetal resin and epoxy resin wherein the improvement comprises the composition defined within the accompanying triaxial diagram of FIG.
 5. 2. A method for coating transformer wire with an insulating coating comprising the steps of:preparing a composition consisting of 30 to 60 weight percent epoxy resin, and a ratio of from 1.10 to 1.90 polyvinyl acetal resin to phenol aldehyde resin; applying the composition to the transformer wire; and heating the coated transformer wire to a first temperature to fuse the coating; and heating the coated wire to a second temperature to cause the coating to react.
 3. The method of claim 2 wherein the step of applying the composition to the wire comprises passing the wire through a coating die.
 4. The method of claim 2 wherein the step of applying the composition to the wire comprises electrostatic coating.
 5. The method of claim 2 wherein the second temperature is higher than the first temperature. 